High Accuracy Painless Method for Measuring Defibrillation Lead Impedance

ABSTRACT

Methods and apparatus for accurately and painlessly measuring the impedance between defibrillation electrodes implanted in a patient utilize a high current test pulse delivered with a sufficiently high current to produce an accurate measurement of the defibrillation electrode impedance while limiting the duration of the test pulse such that the pain sensing cells in the patient do not perceive the test pulse. In one embodiment, the test pulse is generated from the high voltage transformer without storing energy in the high voltage capacitors and is delivered to the defibrillation electrodes in the patient utilizing the high voltage switching circuitry.

CROSS-REFERENCE TO RELATED CASES

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/999,041 filed on Oct. 15, 2007, the disclosure of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to improvements for implantablemedical devices such as devices for delivering defibrillation current toa body. More particularly, the invention relates to an implantablemedical device that determines the impedance between defibrillationelectrodes without causing pain to the body.

BACKGROUND

Implantable cardioverter defibrillators (ICDs) are used to providevarious types of therapy to a cardiac patient, including, for exampledefibrillation. These devices consist of a hermetic housing implantedinto a patient and connected to at least one defibrillation electrode.The housing of the ICD contains electronic circuitry for monitoring thecondition of the patient's heart, usually through sensing electrodes,and also contains the battery, high voltage circuitry and controlcircuitry to generate, control and deliver the defibrillation shocks.Typically, one or more defibrillation leads are connected to circuitrywithin the ICD and extend from the housing to one or more defibrillatorelectrodes proximate the heart. The housing of the ICD may include oneor more defibrillation electrodes conFIG.d on the exterior of thehousing. One example of an ICD is disclosed in U.S. Pat. No. 5,405,363to Kroll et al., the disclosure of which is hereby incorporated byreference.

One important parameter for the effective operation of an ICD device isthe defibrillation electrode impedance. This impedance is indicative ofthe positioning and integrity of the defibrillation leads and/orelectrodes. Electrode impedance is also related to the defibrillationthreshold for a given patient used in setting the energy levels fordefibrillation shocks for that patient. Successful cardiacdefibrillation depends on the amount of energy applied to the cardiactissue by the electrical defibrillation shock, and the energy of thedefibrillation shock is dependent on the electrode impedance of thedefibrillation electrodes through which the defibrillation shock isdelivered.

Determining the impedance between defibrillation electrodes is used indifferent ways when implanting and operating an ICD. One use is to allowa physician to verify that the defibrillation leads and/or electrodeshave not shifted after an initial placement. Another use is to permitthe physician to adjust waveform durations in the event of a significantimpedance change. Still another use is to confirm the viability andsettings appropriate for a defibrillation shock prior to delivering thedefibrillation shock. Thus, it can be seen that accurate knowledge ofthe electrode impedance is important both during implantation andoperation of an ICD device.

Presently, ICD devices periodically measure the impedance across thedefibrillation leads by using a low voltage monophasic or alternatingsquare wave pulse on the order of 10 volts. Most ICD devices use a lowvoltage monophasic pulse that is generated from the battery, rather thanthe high voltage capacitors that are used to generate and deliver adefibrillation shock. This is done both to keep the test shock at alevel that is below the pain or perception level that may be felt by apatient, and to minimize the drain on the battery in order toperiodically supply these test shocks.

With a normal defibrillation shock, the current passed through thedefibrillation electrodes is on the order of ten amperes and severalhundred volts and many charge carriers in the cardiac tissue arerecruited to carry this current. When the cardiac tissue is subjected toa lower current pulse, fewer charge carriers are recruited to carry thelower current. As a result, the impedance of the cardiac tissue inresponse to a lower current pulse increases significantly. For example,a forty ohm (Ω) defibrillation pathway might have an apparent impedanceof over 120Ω with a lower voltage and correspondingly lower currentpulse. This differential behavior of cardiac tissue in response todifferent amounts of current is discussed by Brewer J E, Tvedt M A,Adams T P, and Kroll M W in Low Voltage Shocks Have a SignificantlyHigher Tilt of the Internal Electric Field Than Do High Voltage Shocks,PACING AND CLINICAL ELECTROPHYSIOLOGY Vol. 18, p. 214 (January 1995),the disclosure of which is hereby incorporated by reference.

Because this differential behavior of cardiac tissue is known, currentICD devices using a low voltage pulse to measure the impedance ofdefibrillation electrode will generate a measured value that can be asmuch as three times greater than the actual defibrillation impedanceencountered for a high voltage, high current defibrillation shock.Consequently, current ICD devices typically divide the impedancemeasured in response to a low voltage test pulse by some kind of “fudge”factor (e.g., 2 or 3) to estimate the actual impedance. Unfortunately,the fudge factor is not consistent with all types of leads, electrodes,patients, or changing electrolyte concentrations. Thus, significanterrors are often introduced that may yield inconsistent impedancemeasurements.

One approach to reducing the errors induced by the use of low voltagetest shocks for measuring defibrillation electrode impedance isdescribed in U.S. Pat. No. 6,104,954 to Blunsden. In one embodiment, asquare wave generator is described to generate a test pulse ofapproximately 50 V and 100 Khz. While this approach would somewhatimprove the accuracy of the impedance measurement, unfortunately thisembodiment is completely impractical for an ICD device as the continuouspower requirement to implement this kind of square wave test pulse wouldbe 50 W, an amount which is well above any continuous power supply thatcan be provided by current defibrillation battery technology of an ICDdevice.

In another embodiment, Blunsden describes the use of a higher voltageshock in the range of defibrillation voltages that is delivered from thehigh voltage capacitors in the ICD as a shorter test shock for purposesof measuring defibrillation electrode impedance. The approach has theadvantage testing not only the defibrillation electrodes, but also theoperation of the high voltage switches used to generate biphasic pulsesthat are typically used for defibrillation shocks. While this approachhas the added advantage of exercising the high voltage switches and canaddress some of the errors induced by the use of low voltage testshocks, the approach introduces the possibility of unwanted shocks inthe event of a failure of the high voltage switches or heightenedsensitivity of the patient to larger voltage shocks. The approach alsorequires an increase in the drain on the battery required in order toperiodically charge the high voltage capacitors to deliver thesedefibrillation range shocks for measuring the defibrillation electrodeimpedance where the vast majority of the energy required to charge thehigh voltage capacitors is ultimately wasted.

Defibrillation strength shocks (approximately ten amperes and severalhundred volts) are extremely painful and cannot be given to consciouspatients. Accurately measuring the defibrillation electrode impedance isimportant to effective operation of an ICD device. There is anunfulfilled need to accurately measure the actual impedance betweendefibrillation electrodes while minimizing or eliminating the sensationof pain felt by the patient and not adversely affecting the overallperformance of the ICD device.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods foraccurately measuring the impedance between defibrillation electrodes ofan ICD device. In one embodiment, high current test pulse of a veryshort duration are delivered to determine the impedance betweendefibrillation electrodes. These short duration and high current testpulses are delivered at a voltage of at least about 75V but below thenormal range of defibrillation shocks of 250-800V. Unlike current lowvoltage test pulse techniques, the high current test pulses produce moreaccurate impedance measurements. Because the pulses are shorter than thetime periods required to sense pain by the human pain sensors, the highcurrent test pulses are not perceived and therefore not painful to thepatient.

In one embodiment, a biphasic test pulse is used to measure theimpedance of the defibrillation electrodes. In accordance with thisaspect of the invention, it is recognized that a short biphasic testpulse, with appropriately balanced phases, will have even lessperception than a short monophasic test pulse. This is because thesecond half of the pulse, the negative phase, tends to cancel out thenascent response forming on nociceptor (pain sensing) cells.

One aspect of this invention is the use of short high current testpulses to automatically monitor the defibrillation electrode impedanceand alert the patient and/or physician in the case of a significantdeviation of the impedance from expected values. Another aspect of thisinvention is the use of short high current test pulses to automaticallymonitor the defibrillation electrode impedance and adjust waveformand/or defibrillation vectors in the case of a significant deviation ofthe impedance from expected values.

A further aspect of this invention is the use of lower chargemonophasic/biphasic test pulses on the order of 2 microcoulombs (μC) togive imperceptible but highly accurate impedance measurements. A stillfurther aspect of this invention is the use of lower chargemonophasic/biphasic test pulses on the order of 20 μC to give painlessbut highly accurate impedance measurements, although it may be possiblethat some patients may perceive these 20 μC pulses.

In one embodiment, a capacitor arrangement separate from the main highvoltage defibrillation capacitors is used in connection with the highvoltage inverter/transformer and the high voltage switching circuit toisolate the main high voltage defibrillation capacitors from thedefibrillation electrodes during impedance testing. Unlike the prior arttechniques, this embodiment permits a practical and efficient use of theICD battery to power the test pulses while accomplishing more accurateimpedance measurements. It also serves to insulate the patent from anunintentional shock of normal defibrillation magnitude as the result ofa device failure by ensuring that the main defibrillation capacitors areonly charged when treatment is necessary and are not required to becharged during impedance testing.

BRIEF DESCRIPTION OF THE FIGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 depicts the relationship between voltage and measured impedancefor various shock energies.

FIG. 2 depicts the relationship between the magnitude and duration of amonophasic pulse of electrical current and the threshold where thatcurrent pulse is perceived.

FIG. 3 depicts the relationship between the magnitude and duration of abalanced biphasic pulse of electrical current and the threshold wherethe current pulse is perceived.

FIG. 4 depicts the relationship between the magnitude and duration of amonophasic pulse of electrical current relative to pain and perceptionthresholds.

FIG. 5 depicts the relationship between the magnitude and duration of abalanced biphasic pulse of electrical current relative to pain andperception thresholds.

FIG. 6 illustrates generally a flow chart diagram of steps to accuratelydetermine impedance according to one embodiment of the invention.

FIG. 7 illustrates generally a flow chart diagram of steps to accuratelydetermine impedance according to one embodiment of the invention.

FIG. 8 depicts a conventional ICD output circuit capable of deliveringmonophasic or biphasic pulses to a heart.

FIG. 9 depicts an ICD output circuit with an additional capacitor andswitches to accurately determine impedance according to one embodimentof the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

As seen in FIG. 1 the measured impedance is highly dependent on theimpressed voltage and the current used. The following calculation yieldsthe voltage required to obtain a 10% error in the impedance. As alsoshown in FIG. 1, the measured impedance for a typical defibrillationelectrode system is giving by:

Z=41+(960/V)−(2041/(V̂2))

where V is the impressed voltage. For larger voltages the last term isinsignificant. Thus the measured impedance can be estimated by:

Z=41+(960/V)

where the actual (high voltage) impedance was 39.4Ω. Thus, to calculatethe voltage for a 10% error (approximately 4 ohms), set:

4+39.4Ω=43.4=41+(960/V)

which reduces to 2.4=(960/V ), yielding a value for V of 400 volts. Forthe impedance of 40Ω, this equates to a required current of ten ampereswhich would be extremely painful to a conscious person. However, as showin FIG. 2, even test pulses of one ampere can give significantaccuracies beyond that seen with conventional approaches at lowercurrent levels.

As depicted in FIG. 2, the threshold current for perception of anelectrical pulse is a hyperbolic function of the pulse duration. For thepulses of duration exceeding the “chronaxie” of pain receptors thethreshold current is fairly constant. The chronaxie value for painreceptors shown in FIG. 2 is conservatively valued at 200 microseconds.(The use of a higher value such as 400 microseconds will give moreoptimistic predictions for the benefit of this invention.) However,below the chronaxie pulse duration, the threshold for perception andpain go up inversely with the pulse duration. Thus the perceptionthreshold is about 3 amperes for a 1 microsecond pulse. The typical painthreshold shown in FIG. 2 is at about ten times the perceptionthreshold. The pain threshold has a similar inverse relationship withthe pulse duration.

The typical maximum unipolar pacing output of 10 mA at 1 millisecond orless is also shown. Patients often perceived this level of stimulation.This area is considered by those skilled in the art to be at theperception edge of most patients. The typical high voltage (HV) leadintegrity test level of 100 mA is also depicted at the edge of the painregion.

FIG. 2 also depicts a triangular region (with log-log plotting) which isabove the 10 amperes required for accuracy and also below the currentlevels where the current is perceived. This region is defined by a pulsewith a duration of 0.3 microseconds or less. This is also for pulse oftotal charge less than 2 μC. The result of these calculations is thatthis small region provides for an accurate measurement of the impedancewithout the patient perceiving the shock.

With a biphasic shock the net charge delivered can be set to zerocoulombs by having essentially equal first and second phases. The resultof this is that the perception and pain thresholds are increased by atleast a multiple of five.

In FIG. 3 the increased thresholds are depicted and the pulse durationplotted is the “total” duration of both phases of a biphasic shock.Since this gives a first phase duration of one-half of the total, thethreshold current is multiplied by two; in addition to themultiplication by five from the second phase cancellation effects. Thisresults in the perception and pain threshold currents being multipliedby a factor of ten for short pulses. Thus, for a biphasic pulse, theregion for an imperceptible—yet high accuracy—impedance test is expandedto include pulses with durations out to about 2 micro-seconds. Theallowable total (rectified) charge is increased to about 20microcoulumbs. Thus, the biphasic impedance test waveform is thepreferred embodiment of this invention.

Another significant advantage of this invention is the ability toaccurately measure impedance without affecting the heart. As seen inFIG. 4, the monophasic cardiac stimulation threshold is above that ofperception for pulses shorter than 10 microseconds. This is based on thecardiac chronaxie for large electrode stimulation of about 2milliseconds.

As seen in FIG. 5, the biphasic cardiac stimulation threshold is abovethat of pain for pulses shorter than 30 microseconds. Thus these testpulses can be given without fear of any cardiac effect. To add furthersafety, the pulses may be given synchronized with the QRS complex.

One method of practicing an embodiment of this invention is shown inFIGS. 6 and 7. First, a shock capacitor located in an implantablecardiovascular defibrillator is charged to a potential of below aconventional defibrillation range corresponding to therapeuticdefibrillation shock energies of at least about 5 J, which equate to atypical charge voltage of at least about 250V. Preferably, this will bea charge voltage for the high current test pulse of between about75-250V, and optimally between about 100-150V. Second, the defibrillatorsynchronizes delivery of a test pulse to the QRS complex of the heart.Third a synchronized monophasic shaped pulse of less than 2microseconds, or biphasic shaped pulse of less than 20 microseconds, isdelivered to the heart through the defibrillator electrodes. Fourth, thecurrent level is measured during the pulse. Finally, the voltage isdivided by the current to determine the impedance between thedefibrillation electrodes.

A conventional ICD output circuit is shown in FIG. 8. Battery 100delivers current to transformer primary 102 when switch 104 is closed.When switch 104 is opened a “flyback” current is generated by thesecondaries 106 and 108 which passes through diodes 110 and 112 tocharge main capacitors 114 and 116 which have typical values of 150-300μF each. Voltage sensor 118 monitors the total capacitor voltage andstops the charging when the capacitor bank has achieved the desiredfinal voltage for a therapeutic defibrillation shock which is in therange of 250-800 volts. The defibrillation shock is delivered to theheart 130 through electrodes by briefly (3-6 milliseconds) closingswitches 122 and 128 for a first phase followed by briefly (3-6milliseconds) closing switches 124 and 126 to deliver a reversedpolarity second phase. The signal from current sensor 120 is dividedinto the voltage from sensor 118 to obtain the real-time impedance ofthe electrodes and the heart.

This conventional ICD system can provide accurate impedance measurementsduring a high voltage shock. However, it provides very inaccuratereadings (up to 300% error) when used with conventional lower voltage(10-15 V) test pulses due to the extreme nonlinearity of the resistancein the electrode-electrolyte interface.

By way of example, FIG. 9 depicts one embodiment of the presentinvention. A small (approximately 1 nF to 1 μF in value) capacitor 136is added in parallel with the main capacitors 114 and 116. In normaloperation this small capacitor 136 helps hold a high current test pulsecharge and thus does not hurt volumetric efficiency of the overalldesign significantly. During normal defibrillation operation the newswitches 132 and 134 are closed allowing the small capacitor 136 tocharge in parallel with the main capacitors 114 and 116.

To accurately measure the impedance without the delivery of adefibrillation shock, switches 132 and 134 are opened. This acts toinsulate the main capacitors 114 and 116 from the output circuit and thepatient. The primary winding switch 104 is turned on to saturate thetransformer core. The transformer will typically store about 40 μJ ofenergy in the core. When the switch 104 is opened this energy will betransferred into capacitor 136. Using an illustrative value of 8 nF forthis small capacitor 136, the voltage stored will be 100 volts given byV=√(2E/C). Output switches 122 and 128 are now turned on to deliver atest pulse to the heart 130. The time constant of an 8 nF capacitor anda typical 50Ω shocking lead impedance gives a time constant of 0.4 μs.This will place the test pulse duration well within the zero-to lowperception range. These values are an example embodiment and should notbe read a limiting the scope of this invention. Those skilled in the artwill recognize that the above values may be adjusted to practice theinvention as necessary depending on the electrode lead technology usedand the physical characteristics of the patient.

In another embodiment, the capacitor 136 is chosen to have a value ofapproximately 100 nF. In this embodiment utilizing a somewhat largecapacitance value, the transformer core and switch 104 must then becycled several times in order to bring the voltage of capacitor 136 upto a desired range for the high current test pulse of 75-250V. In oneembodiment, the transformer core and switch 104 are cycled a sufficientnumber of time to generate a charge of approximately 100 volts oncapacitor 136. The output H-bridge is then cycled to provide a veryshort monophasic or biphasic test pulse according to the detailedmethods of this invention already described above.

It will be understood that numerous known techniques for determining theimpedance of a given combination of defibrillation electrodes under testusing the high current test pulse of the present invention may beutilized, such as by measuring current, voltage, capacitive decay,duration, or any combination thereof. It will be understood that thegiven combination of defibrillation electrodes may comprise two or moredefibrillation electrodes, where the electrodes may be situated on oneor more defibrillation leads, patch electrode, housing electrode orstent electrode arrangements positioned within or without the heart.

Finally, while the present invention has been described with referenceto certain embodiments, those skilled in the art should appreciate thatthey can readily use the disclosed conception and specific embodimentsas a basis for designing or modifying other structures for carrying outthe same purposes of the present invention without departing from thespirit and scope of the invention as defined by the appended claims.

1. An automated method of determining defibrillation electrode impedanceof at least a pair of defibrillation electrodes implanted proximate aheart of a patient comprising: applying a high current test pulse havinga current greater than or equal to 1 ampere for a duration less than orequal to 3 microseconds to the defibrillation electrodes; andautomatically determining the impedance between the defibrillationelectrodes.
 2. The automated method of claim 1 wherein applying the highcurrent test pulse utilizes a monophasic test pulse of a duration lessthan or equal to 3.0 microseconds and greater than 0.3 microseconds. 3.The automated method of claim 1 wherein applying the high current testpulse utilizes a biphasic test pulse of duration less than or equal to2.0 microseconds.
 4. The automated method of claim 1 wherein applyingthe high current test pulse utilizes a biphasic test pulse of durationless than 2.0 microseconds and greater than 0.2 microseconds to theleads.
 5. The automated method of claim 1 further comprising:synchronizing the test pulse to a QRS complex of the heart to avoidcardiac stimulation.
 6. The automated method of claim 1 wherein themethod is performed by an implantable cardioverter defibrillator thatincludes a high voltage transformer electrically connected between abattery and a high voltage capacitor configured to selectively deliver atherapeutic defibrillation shock through the defibrillation electrodes,and applying the high current test pulse is performed by the implantablecardioverter defibrillator without charging the high voltage capacitorsystem.
 7. The automated method of claim 6 wherein, applying the highcurrent test pulse is performed by utilizing a smaller capacitor systemelectrically connected to a secondary winding of the high voltagetransformer and a switching arrangement that selectively switches outthe high voltage capacitor system when applying the high current testpulse from the smaller capacitor system.
 8. A method, comprising:providing an implantable cardioverter defibrillator that includes a highvoltage transformer electrically connected between a battery and a highvoltage capacitor configured to selectively deliver a therapeuticdefibrillation shock through the defibrillation electrodes; providinginstructions for implanting the cardioverter defibrillator in a patient,the instructions comprising: attaching at least a pair of defibrillationelectrodes proximate a heart of the patient; using the implantablecardioverter defibrillator to apply a high current test pulse having acurrent greater than or equal to 1 ampere for a duration less than orequal to 3.0 microseconds to the defibrillation electrodes; anddetermining an impedance between the defibrillation electrodes, inresponse to the high current test pulse such that the impedance isdetermined without noticeable pain by the patient.
 9. The method ofclaim 8 wherein applying the high current test pulse utilizes amonophasic test pulse of a duration less than or equal to 3.0microseconds and greater than 0.3 microseconds.
 10. The method of claim8 wherein applying the high current test pulse utilizes a biphasic testpulse of duration less than or equal to 2.0 microseconds.
 11. The methodof claim 8 wherein applying the high current test pulse utilizes abiphasic test pulse of duration less than 2.0 microseconds and greaterthan 0.2 microseconds to the leads.
 12. The method of claim 8 where inthe instructions further comprise: synchronizing the test pulse to a QRScomplex of the heart prior to applying the high current test pulse inorder to avoid cardiac stimulation.
 13. An implantable cardioverterdefibrillator comprising: a high voltage transformer electricallyconnected between a battery and a high voltage capacitor configured toselectively deliver a therapeutic defibrillation shock through at leasta pair of defibrillation electrodes; and means for delivering a highcurrent test pulse having a current greater than or equal to 1 amperefor a duration less than or equal to 3 microseconds to thedefibrillation electrodes; and means for automatically determining theimpedance between the defibrillation electrodes in response to the highcurrent test pulse such that the impedance is determined withoutnoticeable pain by the patient.
 14. The implantable cardioverterdefibrillator of claim 13 wherein the means for automaticallydetermining the impedance between the defibrillation electrodes inresponse to the high current test pulse such that the impedance isdetermined without perception by the patient.
 15. The implantablecardioverter defibrillator of claim 13 wherein the high current testpulse utilizes a monophasic test pulse of a duration less than or equalto 3.0 microseconds and greater than 0.3 microseconds.
 16. Theimplantable cardioverter defibrillator of claim 13 wherein the highcurrent test pulse utilizes a biphasic test pulse of duration less thanor equal to 2.0 microseconds.
 17. The implantable cardioverterdefibrillator of claim 13 wherein the high current test pulse utilizes abiphasic test pulse of duration less than 2.0 microseconds and greaterthan 0.2 microseconds to the leads.
 18. The implantable cardioverterdefibrillator of claim 13 further comprising: means for selectivelydelivering a therapeutic defibrillation shock through the defibrillationelectrodes, and means for applying the high current test pulse by theimplantable cardioverter defibrillator without charging the high voltagecapacitor system.
 19. The implantable cardioverter defibrillator ofclaim 18 further comprising: a smaller capacitor system electricallyconnected to a secondary winding of the high voltage transformer; aswitching arrangement that selectively switches out the high voltagecapacitor; and means for applying the high current test pulse throughthe pair of defibrillation electrodes utilizing the smaller capacitorsystem.